Lost foam casting, also known as evaporable foam casting, is a conventional casting method in which a pattern is formed of an evaporable polymeric foam material having a configuration substantially identical to the part to be cast. The foam pattern is normally coated with a ceramic wash material which prevents metals and reaction and facilitates cleaning of the cast metal part. The pattern containing the wash coating is supported in a mold and surrounded by an unbonded particulate material such as sand. During casting, when the molten metal contacts the pattern, the foam material in various fractions, melts, vaporizes and decomposes with the liquid and vapor products of the degradation passing through the porous wash coating and into the interstices of the sand, while the molten metal replaces the void created by displacement of the foam material to thereby form a cast article identical in shape to the pattern.
The use of lost foam casting is particularly useful when casting large articles of complex configuration, such as cylinder blocks for internal combustion engines. In the past, polystyrene has been most commonly used in producing foam patterns for lost foam casting and polymethylmethacrylate has seen some limited use. In addition, U.S. Pat. Nos. 4,633,929 and 4,773,466 describe the use of polyalkylene carbonate foam in producing iron castings.
Aluminum silicon alloys containing less than about 11.6% by weight of silicon are referred to as hypoeutectic alloys and have seen extensive use in the past. The unmodified alloys have a microstructure consisting of primary aluminum dendrites, with a eutectic composed of acicular silicon in an aluminum matrix.
On the other hand, aluminum silicon alloys containing more than about 11.6% silicon are referred to as hypereutectic alloys and contain primary silicon crystals, which are precipitated as the alloy is cooled between the liquidus temperature and the eutectic temperature. Due to the high hardness of the precipitated primary silicon crystals, these alloys have better wear-resistance than the hypoeutectic alloys, but have a relatively large or wide solidification range. The solidification range, which is the temperature range over which the alloy will solidify, is the range between the liquidus temperature and the invariant eutectic temperature. The wider the solidification range, the longer it will take for an alloy to solidify at a given rate of cooling. For casting purposes, a narrow solidification range is normally desired.
It is also recognized that hypereutectic aluminum silicon alloys are more difficult to cast than hypoeutectic aluminum silicon alloys, because hypereutectic alloys are difficult to "feed", and this casting characteristic worsens as the silicon content is increased.
Hypereutectic aluminum silicon alloys are inherently difficult to cast using lost foam casting processes because of the flotation of the primary silicon crystals during slow cooling, and because of the difficulty of feeding metal shrinkage during slow cooling that results due to the wide solidification range of these alloys. As a further problem, hypereutectic aluminum silicon alloys produced by lost foam casting utilizing polystyrene foam patterns often contain defects resulting from trapped liquid foam transformation products, defects commonly referred to as "liquid styrene defects". These defects appear as elongated rifts, and may extend either partially or through the entire thickness of the casting. It is believed that the "liquid styrene defects" result because the liquid styrene that accumulates on the advancing molten metal front remains a liquid longer than the metal, particularly when two molten metal streams meet in the far reaches of a complex casting, and have lost a significant portion of their initial super heat. Even after solidification, the solidified metal continues to transfer heat to the liquid styrene, eventually causing its evaporation and creating a void in the space previously occupied by the liquid styrene. With castings such as engine blocks which are subjected in use to high internal pressures, leakage can occur through the defects.
In certain cases, repair welding can be utilized to repair visible "liquid styrene defects", but this is an expensive procedure and is not an option where the defects are internal and not visible. Even when the defects do not penetrate the entire casting thickness and thus do not impair the functionality of the casting, the defects greatly degrade the aesthetics of the casting surface and hinder the acceptance of the casting in any market that cannot tolerate a roughened skin appearance.
Numerous attempts have been made in the past to eliminate the "liquid styrene defect". One attempt was to use expanded polystyrene foam of a lower density. Typical expanded polystyrene foam as used in lost foam casting has a bulk density of about 1.6 pounds/cubic foot, and it was thought that by using a polystyrene foam of lesser density, i.e. 1 pound per cubic foot, a lesser volume of decomposition products would be produced, which theoretically could minimize the defects. However, the use of lesser density polystyrene foam did little to eliminate the defects in hypereutectic aluminum silicon casting.
It was also suggested to cast the hypereutectic aluminum silicon alloys at higher temperatures to allow more time for the liquid styrene to be transported out of the casting. Like the use of low density expanded polystyrene foam, the higher casting temperature did not result in a solution for the "liquid styrene defect".
It was also suggested to use a wash coating on the polystyrene pattern which was more porous or permeable. Again, the use of a more porous coating did not reduce the "liquid styrene defects" in casting of the hypereutectic aluminum silicon alloys.
It was also proposed to use heated sand at a temperature of 150.degree. F. which would facilitate more effective wetting and wicking of the liquid styrene into the coating. Again, this was not effective in solving the defect.
In summary there has been a distinct need for a solution to the "liquid styrene defect" which occurs when casting hypereutectic aluminum silicon alloys using polystyrene foam patterns in a lost foam casting process.
It has also been recognized that defects can occur when casting hypoeutectic aluminum silicon alloys using lost foam techniques. The most serious defect, characteristic of the hypoeutetcic aluminum-silicon alloys is the "fold" defect.
This defect, unlike the liquid styrene defect of the hypereutectic aluminum-silicon alloys, basically has carbonaceous, pyrolyzed, decomposed foam products trapped (i.e. sandwiched) between a folded over oxide film and at the surface of the casting does not impair the aesthetics of the casting.